The present invention relates to methods for fabricating packages for microelectronic devices.
In the electronic industry, it is conventional to encapsulate microelectronic devices such as integrated circuits in chip carrier packages or overmolded packages. These packages protect the microelectronic device from the environment, and provide means for electrically and mechanically attaching the microelectronic device in the intended system.
The conventional technique for forming these packages comprises transfer molding. The leadframes are separated into sections for a predetermined number of packages, and microelectronic devices are attached to the leadframes. Transfer molding is a batch process that has been used in the electronics industry for many years, where leadframes are transferred to a molding machine, a mold cavity is formed around the leadframe and device, and plastic is injected into the mold to form the package.
Subsequently, chip carrier type packages can be filled with an encapsulant and then sealed. The leads for plastic packages are then plated. This entails a complex set of processes which can be incompatible with other packaging process steps, and therefore requires more careful and expensive treatment to prevent damage to the sealed package.
Additionally, in conventional molding of plastic packages, when the mold is closed around the leadframe, there are spaces between the leads from which the plastic can leak from the mold. In order to prevent this, a section of the leadframe called a dambar acts as a dam to prevent the leakage of plastic. After the plastic package has been set and cured, the dambars are cut away with a series of precision cutting tools. This is conventionally done at the stage when the packages, typically in strip form, are subsequently separated into individual packages. These cutting tools are expensive because of the need for precision and tight tolerances in the microelectronic devices.
Despite these disadvantages, it is desirable to create a die package for a semiconductor device which is essentially a molded package. This is because a molded package is sturdy and cost effective. Additionally, the electronic industry is accustomed to the molded package in that the product design and assembly processes are set up to use molded die packages. It would be advantageous to have improved methods for forming the plastic packages.
In one aspect, this invention provides a method of forming microelectronic packages of the type including a microelectronic device, a receptacle formed of an insulative material and defining a cavity for receipt of the microelectronic device, and a leadframe formed of a conductive material and facilitating electrical and mechanical connection of the microelectronic device to an external circuit.
According to the invention, the packages are formed in a continuous process comprising: moving a continuous strip of an electrically conductive substrate material along a feed path to a removal station and removing material from the continuous strip to define a series of successive connected leadframes from the remaining material with each leadframe including a series of leads; at a forming station along the feed path, forming a receptacle of electrically insulative material on the substrate strip at a respective leadframe, and attaching a device in the cavity at an attachment station. As such, a continuous strip of successively connected cavity packages, each containing a device are formed. This method has the advantage of providing a continuous production of packages resulting in a savings of fabrication time and material.
Another aspect of the invention is to include one or more additional processing steps along the feed path. The process can be extended to include: a wire bonding step whereby the microelectronic device is electrically connected to the leads via a plurality of wires; a liquid fill station where each cavity is filled with a liquid encapsulant; a curing station for setting the liquid encapsulant; a sealing station for sealing the package with a lid; a marking station for labeling each package; and a separation station for the separation of the continuous strip of packages into discrete strips containing a desired number of packages. The continuous process of the invention facilitates selected addition of the recited steps depending upon the requirements of a particular package design or the availability of particular production equipment.
Other objects, advantages and applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings.